The U.S. global positioning system (GPS) is a type of GNSS system including a constellation of space vehicles (e.g., satellites) that orbit the earth to provide navigation and positioning signals to GPS/GNSS receivers or navigation devices. Millions of GPS/GNSS receivers or navigation devices capable of receiving and using GPS/GNSS signals are in use by the general public and government entities.
A GPS/GNSS receiver calculates its position by precise timing of the signals sent by GPS/GNSS space vehicles. Each space vehicle continually transmits navigation messages that include (1) the time the message was transmitted and (2) space vehicle position at the time of message transmission. The receiver analyzes the navigation messages received from a minimum of four GPS/GNSS space vehicles. The receiver determines the transit time of each navigation message and computes the respective distances to each space vehicle using the speed of light. Knowing the distance from the receiver to each space vehicle and each space vehicle's respective position, the receiver determines its position in three absolute spatial coordinates and one absolute time coordinate.
Precise timing is critical to high precision tracking and navigation in GPS/GNSS systems. As such, GPS/GNSS space vehicles utilize high precision atomic frequency standards (AFS), such as rubidium atomic clocks, for timing. AFSs can exhibit various clock anomalies that can introduce significant errors in GPS/GNSS navigation and tracking if left undetected.